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VRF templates : How to fix these error: "Gross Rated Total Cooling Capacity [W] will be limited by the maximum rated volume flow per rated total capacity ratio"

asked 2017-05-22 13:17:41 -0500

melanie gravatar image

updated 2019-04-18 21:32:38 -0500

Alexis gravatar image

In all the Cooling Coils of the Variable Refrigerant Flow System is happening the same error:

* Warning * SizeDXCoil Coil:Cooling:DX:VariableRefrigerantFlow TU7 VRF DX COOLING COIL * ~~~ * ...Gross Rated Total Cooling Capacity [W] will be limited by the maximum rated volume flow per rated total capacity ratio. * ~~~ * ...DX coil volume flow rate ( m3/s ) = 2.347873 * ~~~ * ...Requested capacity ( W ) = 15364.597 * ~~~ * ...Requested flow/capacity ratio ( m3/s/W ) = 1.528E-004 * ~~~ * ...Maximum flow/capacity ratio ( m3/s/W ) = 6.041E-005 * ~~~ * ...Adjusted capacity ( W ) = 38865.635

And as you can see, automatically for to adjust the ratio, the capacity of the coils are almost triplicating.

Answer to Rraustad:
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Does anyone have an answer or a lead for this issue ? I am facing the same issue with the VRF templates and autosize for all the fields.
I read this post but it didnt help me : link text

Alexis gravatar imageAlexis ( 2019-04-05 05:28:57 -0500 )edit

@Alexis, did changing the SAT in the Sizing:Zone object help?

rraustad gravatar imagerraustad ( 2019-04-17 11:45:57 -0500 )edit

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answered 2019-04-05 07:15:28 -0500

I assume you are autosizing the coils otherwise you could have specified the correct flow ratio. In the Sizing:Zone objects, the supply air temperature is specified which is used to determine the zone air flow rate. During sizing this supply air temperature, along with the zone temperature, is used to calculate a zone air flow rate given the zone load, for each time step. The air flow rate at the maximum load is used as the design load. These parameters are used to size the coil. If the Sizing:Zone supply air temperature is too high (i.e., close to the zone temperature) the zone air flow rate will also be high. A reasonable temperature difference for DX equipment is 11C so the supply air temperature should be around T = zone temp - 11 C.

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Thank for the answer. I tried but I still have these warnings.
For info, the difference between the 'Requested capacity' and the 'Adjusted capacity' is around 10 % so it gives me a safety factor but I would prefer to understand how to avoid this issue.
And I have these warnings only with the VRF templates. I have an other thermal zone with PTAC and I don't get this warning

Alexis gravatar imageAlexis ( 2019-04-09 21:15:04 -0500 )edit

Did you try lowering the Sizing:Zone supply air temperature to increase the autosized zone air flow rate?

rraustad gravatar imagerraustad ( 2019-04-15 11:58:37 -0500 )edit

I tried, [lease Check above my screenshot. But I don't really know how to read and understand the results. Which one I need to choose to size my indoor unit ?

Alexis gravatar imageAlexis ( 2019-04-17 03:03:09 -0500 )edit

So when you adjusted the Sizing:Zone supply air temperature did you see a change in the flow per capacity ratio in the warnings? Is the requested flow ratio approaching the limit? Or diverging from the limit? You might run this again with original inputs and these and then check the warnings in each case to see what is happening.

rraustad gravatar imagerraustad ( 2019-04-17 08:42:57 -0500 )edit

And I may have led you in the wrong direction. Zone air flow = Q/(Cp*deltaT) so as deltaT (Tzone - Tsupply air) increases the zone flow decreases. I think you wanted to increase flow and if so would need to raise the supply air temperature (decrease deltaT).

rraustad gravatar imagerraustad ( 2019-04-17 08:45:51 -0500 )edit

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Asked: 2017-05-22 13:17:41 -0500

Seen: 328 times

Last updated: Apr 18